252 Dr. R. H. Traquair — On Chondrosteus acipenseroides. 



other specimen, though the existence of a cartilaginous one may be 

 safely assumed considering the large space between the lower 

 extremity of the hyomandibular and the articulation of the lower 

 jaw. 



The ceratohyal (c. h. Figs. 2, 3, and 5) is also very easily recog- 

 nizable, and requires no special description. 



Fig. 3.— Head of Chot,drosteus (British Museum, P. 2048), seen obliquely from 

 below, showing the position of the mouth and the symphyses of both maxillBe and 

 mandibles. The suboperculum and branchiostegal rays are somewhat injured. 



Opercular apparatus. — The opercular flap is principally supported 

 by a large, broad, somewhat irregularly rlaombic plate (s. op. Figs. 

 2, 3, and 5), overlapping, with its anterior superior angle, the 

 posterior inferior part of the hyomandibular, and leaving a consider- 

 able space between its upper concave margin and the edge of the 

 cranial shield. This is the bone which has hitherto been called 

 " operculum " in Chondrosteus, and it certainly corresponds exactly 

 in position to the so-called operculum in Polyodon. It is, however, 

 equally clear that it corresponds also in position, as well as in general 



